so I don't think I'd be too rough on the 105 wheel set.
The only 105 component in the wheelset would be the hubs. You don't need to worry about harming them, they'll be fine. The M770-series DeoreXT hubs
are quite nice, and with their aluminum axles they might even be lighter than 105, but as
Port said, the slight weight differences aren't going to be a big deal. The Crosscheck is unusual in that it would accept either 130mm (105) or 135mm (XT), so it's up to you.
As for the rest, you might want to compare the gearing options and think that aspect through. Note that XT cranks are available in both mountain (22-32-44) and trekking (26-36-48). XT has thick forged chainrings (except the new carbon-fiber 32-tooth ring with steel teeth, found only on the M770-series cranks). 105 has stamped rings... compare a 105 crank to Ultegra and you'll see the difference. Personally, I like the beefier rings, but it might not be a big deal to everyone, and I still think the best plan is to see which route suits your gearing needs best. Do you need a higher high gear than a 48 x 11? Then don't get XT. Do you need a lower low gear than a 30 x 28? Then don't get 105, unless you want to either pony up for an IRD cassette or stuff an even smaller granny ring onto it.